This invention relates to pole-changeable, three-phase windings of the phase-modulated type.
There are three ways in which a continuous sinusoidal wave, or carrier wave, can be modulated: amplitude modulation, phase modulation and frequency modulation. These three modulation modes are familiar in telecommunications practice, but the theory applies also to rotary electrical machines, where the continuous wave concerned is the travelling wave of ampere-conductors, or current sheet, or magnetomotive force, around a polyphase winding.
Frequency modulation provides speed-changing through variation of the supply frequency. Amplitude modulation and phase modulation are applied to the space distribution of a polyphase winding to bring about speed change by pole-number changing.
Pole-amplitude modulation (P.A.M.), invented by Professor Rawcliffe and others, effects modulation of pole-amplitudes in a phase-winding by reversal of direction of current flow in one half phase-winding with respect to the other half. The method can thus be implemented, in a three-phase winding, using only six winding terminals and a simple changeover switch.
Phase modulation is the oldest known method because early proposals for speed-changing motors, requiring the switching of winding parts into different supply phases for different speeds, are in fact phase modulating methods. These proposals found little practical use, however, because they necessitated a large number of winding terminals and complicated switching.
Recently, that is to say a little more than a decade ago, there have been proposed a class of phase-modulated, pole-changeable motors capable of being implemented with six-terminal, changeover switching. An example is described in British Patent specification No. 1,484,193. The method there disclosed requires a three-phase winding with some coils which are connected in circuit for one of the alternative pole-numbers and omitted for the other. More generally, the method may require a number of such coils, some of which number are used only for one pole-number and others of which number are used only for the other pole-number.
Winding arrangements are disclosed in the above-quoted patent specification enabling these methods to be implemented with six-terminal, changeover switching.
Given an alternating current supply of fixed frequency, as is the common practical case, speed-changing involves pole-number changing and this necessitates a choice between the pole-amplitude modulation and the phase-modulation methods.